1. Field of the Invention
The present invention relates to an optical sensor.
2. Description of the Related Art
Jpn. Pat. Appln. KOKAI Publication 57-141604, for example, discloses an optical fiber. As shown in FIG. 7A, the optical fiber 101 has a core 101a, a cladding 101b covering the core 101a, and a light absorbing part 101c which is provided in the cladding 101b. 
How light propagates in the optical fiber 101 will be explained.
In this case, the optical fiber 101 may extend straight as shown in FIG. 7A, all of light 103a is guided in the axial direction of the optical fiber 101. The light 103b propagating at a first angle to the axial direction is absorbed in the light absorbing part 101c. The light 103c propagating at a second angle to the axial direction is not absorbed in the light absorbing part 101c, and is completely reflected by the cladding 101b and is guided forwards.
As shown in FIG. 7B, when the optical fiber 101 may be bent at the light absorbing part 101c, light beams 103, 103b, and 103c propagate toward the light absorbing part 101c. As a result, the light beams 103, 103b, and 103c are absorbed in the light absorbing part 101c, and will no longer propagate.
The light intensity guided is thus controlled.
Such the optical fiber 101 is used a curvature measuring device 110 as shown in FIG. 7C which detects the dislocation of the optical fiber 101 and which is a representative optical sensor. The curvature measuring device 110 shown in FIG. 7C has an optical fiber 101 shown in FIG. 7A and laid along a rail 111, a laser light source 113 connected to one end of the optical fiber 101, and a photoelectric transducer device 115 connected to the other end of the optical fiber 101. The optical fiber 101 is bent to match the curvature of the rail 111. In proportion to the curvature of the rail 111, the light decreases in intensity as it propagates in the optical fiber 101 from the laser light source 113 to the photoelectric transducer device 115. The photoelectric transducer device 115 measures the decrease in the light intensity in compliance with the curvature. Therefor the curvature of the rail 111 and the downward flexure of the rail 111 can be measured.
The light absorbing part 101c, i.e., characteristic changing part, is positioned as shown in FIG. 7A. In the optical sensor shown in FIG. 7C, the optical fiber 101 is provided such that the light absorbing part 101c is located for example, on the outer circumference face side of the bending rail 111. Thus, the optical fiber 101 including the light absorbing part 101c is provided in accordance with the direction in which the rail 111 is bent.
However, if the optical fiber 101 is provided in, for example, a small high-precision device that is long, thin and flexible, it will be twisted as the high-precision device is twisted and bent, the optical fiber 101 is twisted. Consequently, the characteristic changing part is dislocated, rendering it difficult to detect the state of the high-precision device.
This invention has been made in view of the above. An object of the invention is to provide an optical sensor in which the characteristic changing part can be reliably positioned and the statuses of the components can be detected accurately and easily.